Although most often associated with Christmas, multicolored holiday lights are used year-round as an appealing decorative accent. Today, LED lights are as ubiquitous as incandescent lights, with manufacturers touting their low power requirements, high efficiency, and bright color. But how do LEDs compare with incandescent decorative lights? Here's what we discovered when measuring the relative irradiance of several multicolored light strings.

Experiment Materials and Spectrometer Setup

We tested one string of 20 multicolored incandescent lights and two strings of multicolored LED lights - one was a new, 10-light string and the other was a slightly used, 20-light string. The incandescent lights were purchased at a local pharmacy for about $4 and the newer LEDs for $5 (for half as many lights). All the lights were manufactured in China.

Our measurement setup comprised a spectrometer (200–1025 nm), an integrating sphere, an optical fiber, and spectroscopy software. We used a radiometrically calibrated light source as the reference and measured the incandescent lights and newer LED lights straight from their packaging and the older LEDs on a "tree" fashioned out of laboratory clamps and fittings.

Measurement Results

The incandescent lights that we measured had broadband spectra with very strong intensity in the shortwave NIR region >700 nm (Figure 1). The light at these wavelengths produces heat, which makes the bulb warm during operation.

Figure 1: In incandescent lights, heat results from the spectral output at wavelengths >700 nm.

The LED light strings that we measured had narrowband emission with no intensity in the shortwave NIR region >700 nm, which allows the bulbs to remain cool to the touch even after long periods of operation. In addition to producing less heat, the LEDs are more energy efficient than incandescent lights.

Generally, the LEDs produced brighter colors than the incandescent lights, especially in the blue and green bulbs (Figure 2).

Figure 2: The LED bulbs had strong output for the colors tested, especially at blue and green wavelengths.

What Can Relative Irradiance Measurements Reveal?

The spectrometer setup used for these measurements showed good results for the incandescent and LED lights tested, providing some insight into the differences between the two types. Variability in performance between the LED strings also was observed. Our experiment suggested that the LEDs perform very well spectrally, although to make an assessment of cost effectiveness versus incandescent lights would require additional testing and analysis.

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